1. Field of the Invention
The present invention relates to masks (or reticles) and other devices for use in microlithographic techniques, especially for use in forming contact holes in semiconductor products with improved depth of focus. The invention may be used to form isolated contact holes, arrays of contact holes and other structures. The present invention also relates to a method of determining a desired three-tone pattern for a microlithographic mask. The method, which may be performed on one or more programmed microprocessors, may involve the selection of one set of dimension data out of a plurality of dimension data sets, where the dimension data sets correspond to different mask patterns, and the selected set is the one that provides the greatest depth of focus.
2. Description of Related Art
A semiconductor device can be fabricated by photolithography, in which light is transmitted through a patterned mask (or reticle). The pattern on the mask is exposed on a layer of photoresist to form the desired feature or features in the semiconductor device. Examples of such features are isolated contact holes and contact holes formed in closely packed arrays. In certain circumstances, it is desirable to make contact holes with very small critical dimensions. The xe2x80x9ccritical dimensionxe2x80x9d is typically the diameter of the hole in the plane of the surface of the semiconductor device. Some devices require contact holes with critical dimensions that are less than the wavelength of the light that is used to expose the photoresist. A dimension that is less than the wavelength of the exposing light is referred to as a xe2x80x9csub-resolutionxe2x80x9d dimension.
A number of binary and phase-shifting masks have been proposed in the prior art. Such masks are shown, for example, in U.S. Pat. No. 6,114,071 (Chen et al.), U.S. Pat. No. 6,077,633 (Lin et al.), U.S. Pat. No. 6,022,644 (Lin et al.) and U.S. Pat. No. 5,707,765 (Chen). There is still a need in the art, however, for a three-tone mask (or other multi-tone mask) that can form holes and other structures with small critical dimensions and minimum side-lobing and with improved depth of focus. Depth of focus is especially important in connection with the formation of small structures in non-flat surfaces. Where the wafer is not flat, it may be necessary to image a pattern at different distances from the lithography system with essentially the same fidelity. In addition, it may be necessary to allow for wafer positioning in the system, wafer curvature, focal plane curvature, etc.
Moreover, there is a need in the art for an economical method of making three-tone masks (or other multi-tone masks) for use in the formation of small critical dimension features with minimal side-lobing and large depth of focus.
The present invention relates to a microlithographic mask for forming a sub-resolution feature in photoresist with improved depth of focus. As noted above, the term xe2x80x9csub-resolutionxe2x80x9d means that the critical dimension of the feature formed in the photoresist is less than the wavelength of the exposing light. According to one aspect of the invention, the mask has a three-tone structure, with a layer of transparent material, a layer of attenuating phase-shifting material overlying the transparent material, and a layer of light-obstructing material (i.e., opaque material and/or partially transmissive material) overlying the phase-shifting material. In a preferred embodiment of the invention, the layer of attenuating phase-shifting material is located between the transparent material and the light-obstructing material. The present invention should not be limited, however, to the specific features of the preferred embodiments shown and described in detail herein.
According to another aspect of the invention, opaque material and the attenuating phase-shifting material are patterned to form a square transparent opening, one or more partially transmissive assist features, which may include a rectangular frame, and an opaque frame and/or background. The opaque frame may be located at the edge of the opening (interposed between a partially transmissive assist feature and the transparent opening). Alternatively, opaque squares, triangles or other polygons can be placed at the comers (or inside) of a partially transmissive frame to control the exposure pattern of the light that is transmitted through the partially transmissive frame.
The transparent material may be quartz or another suitable material. The partially transmissive material causes a phase shift (e.g., 180xc2x0 or an odd multiple thereof) relative to the light transmitted through the transparent material. The partially transmissive material also attenuates the phase-shifted light relative to the non-phase-shifted light. The transmissivity of the partially transmissive material relative to the transparent material may be in the range, for example, of from about 6% to 100%, more preferably from about 8% to about 24%. The partially transmissive material may be, for example, MoSi. The opaque material may be a metal such as chrome, and other suitable materials may be employed as desired.
The present invention may be used to form a variety of microlithographic features. The invention is especially well suited, however, for forming a contact hole that has a large aspect ratio of depth to width. The invention is also well suited to forming other structures where a large depth of focus is desirable, such as microlithographic features on substantially non-flat surfaces. According to one aspect of the invention, improved depth of focus is achieved by providing sub-resolution assist features that are patterned in the opaque material and/or the partially transmissive material.
The present invention also relates to masks for forming regular and asymmetric arrays of features, such as arrays of high aspect ratio contact holes. According to one aspect of the invention, elongated assist bars (of partially transmissive material and/or opaque material) are employed to interact with an array of transparent openings. According to another aspect of the invention, phase-shifting assist features are nested within transparent bars.
The present invention also relates to a method of forming elliptical holes and other structures with small critical dimensions and improved depth of focus. The holes may be isolated structures or they may be formed in a dense array.
The present invention also relates to a method of making a multi-tone microlithographic mask. The method, which may be performed at least in part on a digital microprocessor, includes the steps of: (1) providing sets of dimension data representative of multiple mask patterns; (2) for each set of dimension data, calculating feature dimension data as a function of optical conditions; and (3) for a desired optical condition, identifying the sets of dimension data that correlate to feature dimension data within desired limits. If desired, the method may also include the step of (4) selecting the one identified set of dimension data that achieves the smallest change in critical dimension between a zero defocus condition and a maximum considered defocus condition.
In a preferred embodiment of the invention, steps (1) and (2) are performed using a computer programmed with PERL/solid-c imaging software. Steps (3) and (4) may be performed using Visual BASIC/Excel software. As noted above, however, the present invention should not be limited to the specific features of the preferred embodiments.
The dimension data can include the widths of transparent openings and the corresponding dimensions of the opaque and partially transmissive assist features. There may be one set of such dimension data for each pattern under consideration. The limits considered in step (3) may include the critical dimension for the exposed feature, the allowable (or desirable) ellipticity, the absence of sidelobes, log-slope, etc. These limits operate to exclude patterns that do not form acceptable features at the desired operating conditions. Once a desired pattern is determined, the pattern is formed in layers of deposited partially transmissive and opaque materials to form the finished mask. As noted above, the two upper layers of the mask may be deposited on a layer of transparent quartz.
These and other advantages and features of the invention will be more readily understood from the following detailed description of the invention which is provided in connection with the accompanying drawings.